To maximize the transmission capacity of an optical fiber transmission system, a single optical fiber may be used to carry multiple optical signals in what is called a wavelength division multiplexed system (hereinafter a WDM system). The multiple optical signals may be multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on separate wavelengths referred to as channels. Modern WDM systems have a high traffic capacity, for example, a capacity to carry 96 channels or more at 10 gigabits per second (hereinafter Gb/s) or more.
The optical fiber transmission system may include a relatively long trunk fiber segment that may be terminated at a transmitting and/or receiving trunk terminal. The optical fiber transmission system may further include one or more branching units situated along its trunk. Each branching unit (BU) may be connected to a branch fiber segment that terminates in a transmitting and/or receiving branch terminal. Each BU may include one or more optical add/drop multiplexers (OADM). Channels may be added to and/or dropped from the trunk fiber segment of the optical transmission system via the OADMs.
When information signals are transmitted over long distances, one or more amplifiers are provided to compensate for signal attenuation. The amplifiers used in some WDM systems (e.g., undersea systems) cannot easily be modified once installed and are initially configured to support a fully loaded link (e.g., 96 channels, each channel carrying 10 Gb/s). In general, it may be desirable that the power per channel be sufficient to provide an adequate signal-to-noise ratio in the presence of the amplified spontaneous emission (ASE) noise from the amplifiers, necessitating a high amplifier total output power for systems with high fully-loaded capacity. The amplifiers may thus be configured to provide an optical output signal at a nominal total optical power.
The nominal amplifier output power level may be insensitive to the power at the input of the amplifier. As the amplifier input power varies over a wide range, the total amplifier output power may change very little around the nominal output power level. As additional channels are added, e.g. at a branching unit, the optical output power per channel may decrease. As channels are dropped, the optical output power per channel may increase.
Optical signals, while propagating through optical fibers, can experience nonlinear interaction. At sufficiently high values of optical power (e.g., more than 1 mW per channel), the optical signal may experience more distortion than at low optical powers (e.g., less than 1 mW per channel) which results in transmission penalty. Therefore, when channels are dropped, e.g., at a branching unit, the value of optical channel power may increase, and network communication performance may suffer. Partial channel loading of a chain of optical amplifiers may result in undesirable noise accumulation in parts of the transmission band and gain reshaping effects that also degrade channel performance.
For example, in the case of a fiber fault, e.g., a cable cut, or a disconnect of a transmitter, a network may lose its designed uniform loading due to the absence of one or more signals and lower power on the channels that carry those signals. Depending on the location of the fault, ASE noise may or may not substitute for the lost signals. Additional ASE noise may also penalize signal-to-noise ratio (SNR) of the remaining signals. If the fault is in a trunk segment and the channel loading is not uniform, signals added at a branching unit may be penalized by the non-uniform loading on the trunk segment. Similarly, if the fault is in a branch segment and the channel loading is not uniform, signals on the trunk segment, passing through the branching unit, may be penalized. It may therefore be desirable to configure a branching unit for fault tolerance to protect signals on a “healthy” path from penalties caused by a fault on another path.
In an undersea optical fiber transmission system, for example, a branching unit may be deployed at remote locations, e.g., on an ocean floor. The branching unit may be configured, at deployment, for adding and/or dropping signals occupying particular channels. The specific configuration may depend on predicted and/or anticipated traffic. The actual traffic may vary from the prediction. Modifying a branching unit, post-deployment, may involve relatively significant cost. It may therefore be desirable to provide a flexible branching unit that may be reconfigured according to, e.g., actual traffic, without modifying the branching unit.